The present invention relates to a phase synchronizing circuit for reproducing a synchronizing carrier wave from a multi-phase PSK (phase-shift-keying) modulated carrier signal, and more particularly to a phase synchronizing circuit capable of efficiently varying the carrier phase.
The reverse modulation system has been known for use in reproducing a synchronizing carrier wave from an N-phase (N = 2.sup.n, n being a positive integer) PSK-modulated carrier signal. This system comprises: a voltage controlled oscillator; an N-phase phase demodulator receiving an N-phase PSK-modulated signal and an output signal from the voltage controlled oscillator; an N-phase phase modulator with one input terminal connected to the output of the demodulator; a first delay circuit connected between the N-phase PSK-modulated signal input terminal of the demodulator and a second terminal of the modulator; a phase detector with one input terminal connected to the modulator output; a second delay circuit and a variable phase shifter connected between the output terminal of the voltage controlled oscillator and a second input terminal of the phase detector; and a low-pass filter connected between the output terminal of the phase detector and the input terminal of the voltage controlled oscillator. Normally, in such systems, the delay time interval provided by the first delay circuit is determined to be equal to the delay time over the path from the modulated signal input terminal of the demodulator to the input terminal of the modulator via the demodulator circuit. Similarly, the delay time T.sub.1 over the path from the output terminal of the voltage controlled oscillator to the second input terminal of the phase detector via the second delay circuit and the variable phase shifter is determined to be equal to the delay time T.sub.2 over the path from the modulated signal input terminal of the demodulator to one input terminal of the phase detector via the first delay circuit and the modulator. To establish phase synchronism in the system, there must be an exact +90.degree. or -90.degree. phase relation between a reproduced carrier wave supplied to one input terminal of the phase detector and an output carrier wave from the voltage controlled oscillator to the other input terminal of the phase detector. In practice, however, it is hardly possible to provide the first and second delay circuits and the modulator with constant delay times; generally, a variable phase shifter is used to establish the necessary phase relation for the system. Ideally, the variable phase shifter should be capable of providing a phase shift through 360.degree. for the carrier frequency. Such phase shifter, however, necessitates an extra space in the circuit. In a practical system, the variable phase shifter is responsible for a phase shift through approximately 100.degree., and the rest of the phase angle to be shifted is managed by varying delays available on the first and second delay circuits. Recently, with the increase in the quantity of data transmitted, the clock frequency used, for example, in millimeter wave communications, has become as high as 200 MHz to 400 MHz, and the carrier frequency has accordingly become high, e.g., 1.7 GHz in millimeter wave communications. In such a system, the sum of the delay T.sub.1 and T.sub.2 is great relative to the period of 1 cycle of the carrier frequency; for example, in a millimeter wave transmission system, T.sub.1 + T.sub.2 = 28 ns in the phase synchronizing circuit and one cycle of the carrier frequency is 0.59 ns where the shifted phase of the carrier wave vs. the sum of the delays is as great as 17,084.degree. (i.e. (28 .times. 360/0.59) = 17,084.degree.). As a result, the phase of the phase synchronizing circuit largely varies with changes in the ambient temperature, and the synchronous pull-in center frequency largely varies with temperature. This problem can be solved in a manner such that the phase synchronizing circuit is formed in the surface of a substrate of homogeneous material (e.g., aluminum oxide ceramic substrate and sapphire substrate) and thus phase variation in the phase synchronizing circuit is reduced. In practice, however, it is extremely difficult to modify the delay time of the delay circuits and fabricate a 360.degree. variable phase shifter on a substrate of homogeneous material. This has made it extremely difficult to establish a proper phase position for the phase synchronizing circuit.